1. Field of the Invention
The present invention relates generally to wireless communication systems and, more specifically, to the transmission of acknowledgement information in an uplink of a communication system.
2. Description of the Related Art
A communication system includes a DownLink (DL) that conveys transmission signals from a Base Station (BS or NodeB) to User Equipments (UEs), and an UpLink (UL) that conveys transmission signals from UEs to the NodeB. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other equivalent terminology.
The UL supports the transmission of data signals carrying information content, control signals providing control information associated with the transmission of data signals in the DL, and Reference Signals (RSs), which are commonly referred to as pilot signals. The DL also supports the transmission of data signals, control signals, and RSs.
UL data signals are conveyed through a Physical Uplink Shared CHannel (PUSCH) and DL data signals are conveyed through a Physical Downlink Shared CHannel (PDSCH).
In the absence of a PUSCH transmission, a UE conveys UL Control Information (UCI) through a Physical Uplink Control CHannel (PUCCH). However, when there is a PUSCH transmission, a UE may convey UCI together with data through the PUSCH.
DL control signals may be broadcast or sent in a UE-specific nature. Accordingly, UE-specific control channels can be used, among other purposes, to provide UEs with Scheduling Assignments (SAs) for PDSCH reception (DL SAs) or PUSCH transmission (UL SAs). The SAs are transmitted from the NodeB to respective UEs using DL Control Information (DCI) formats through respective Physical DL Control CHannels (PDCCHs).
A UE may be configured by the NodeB through higher layer signaling, such as Radio Resource Control (RRC) signaling, a PDSCH Transmission Mode (TM). The PDSCH TM is associated with a respective DL SA and defines whether the PDSCH conveys one data Transport Block (TB) or two data TBs. The UE may also be configured by NodeB communication, through higher layer signaling, over multiple DL cells for the potential reception of respectively multiple PDSCHs.
The UCI includes ACKnowledgment (ACK) information associated with a Hybrid Automatic Repeat reQuest (HARM) process (HARQ-ACK). The HARQ-ACK information may consist of multiple bits corresponding to positive ACKs for TBs correctly received by the UE, or Negative ACKnowledgements (NACKs) for TBs incorrectly received by the UE. A NACK may also be generated by a UE in response to the absence of a TB reception.
FIG. 1 illustrates a conventional PUSCH transmission structure.
Referring to FIG. 1, the Transmission Time Interval (TTI) is one subframe 110, which includes two slots. Each slot 120 includes NsymbUL symbols used to transmit data signals, UCI signals, or RSs. Each symbol 130 includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The transmission in one slot 120 may be either at a same or at a different BandWidth (BW) than the transmission in the other slot. Some PUSCH symbols in each slot are used to a transmit RS 140, which enables channel estimation and coherent demodulation of the received data and/or UCI signals.
The transmission BW includes frequency resource units, referred to herein as Physical Resource Blocks (PRBs). Each PRB includes NscRB sub-carriers, or Resource Elements (REs), and a UE is allocated MPUSCH PRBs 150 for a total of MscPUSCH=MPUSCH·NscRB REs for the PUSCH transmission BW.
The last subframe symbol may be used for transmitting a Sounding RS (SRS) 160 from one or more UEs. The SRS provides the NodeB with an estimate of the channel medium the respective UE experiences over the SRS transmission BW. The SRS transmission parameters are configured to each UE by the NodeB through higher layer signaling.
In FIG. 1, the number of subframe symbols available for transmission of data or UCI signals is NsymbPUSCH=2·(NsymbUL−1)−NSRS, where NSRS=1 if the last subframe symbol is used for SRS transmission and NSRS=0 otherwise.
FIG. 2 illustrates a conventional transmitter for transmitting data information and HARQ-ACK information in a PUSCH.
Referring to FIG. 2, encoded HARQ-ACK bits 210 are inserted by puncturing encoded data bits 220 at a puncturing unit 230. The Discrete Fourier Transform (DFT) is then performed by a DFT unit 240. The REs for the PUSCH transmission BW are selected by a sub-carrier mapping unit 250 as instructed by a controller 255. Inverse Fast Fourier Transform (IFFT) is performed by an IFFT unit 260, CP insertion is performed by a CP insertion unit 270, and time windowing is performed by a filter 280, thereby generating a transmitted signal 290. For brevity, the encoding and modulation processes and additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated.
The PUSCH transmission is assumed to be over a single cluster 295A or over multiple clusters 295B of contiguous REs in accordance to the DFT Spread Orthogonal Frequency Division Multiple Access (DFT-S-OFDMA) method for signal transmission.
FIG. 3 illustrates a conventional receiver for receiving a transmission signal as illustrated in FIG. 2.
Referring to FIG. 3, an antenna receives a Radio-Frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) which are not shown for brevity, a received digital signal 310 is filtered by a filter 320 and the CP is removed by a CP removal unit 330. Subsequently, the receiver unit applies a Fast Fourier Transform (FFT) by an FFT unit 340, selects the REs used by the transmitter by sub-carrier de-mapping by a sub-carrier demapping unit 350 under the control of a controller 355. Thereafter, an Inverse DFT (IDFT) unit 360 applies IDFT, an extraction unit 370 extracts the HARQ-ACK bits, places erasures at the respective REs for the data, and finally obtains data bits 380.
The RS transmission is assumed to be through a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence. Orthogonal multiplexing of CAZAC sequences can be achieved by applying different Cyclic Shifts (CSs) to the same CAZAC sequence.
Assuming for simplicity that the PUSCH conveys a single data TB, then for HARQ-ACK transmission in the PUSCH, a UE determines the respective number of encoded symbols Q′ as shown in Equation (1).
                              Q          ′                =                  min          (                                    ⌈                                                O                  ·                                      β                    oddset                                          HARQ                      -                      ACK                                                                                                            Q                    m                                    ·                  R                                            ⌉                        ,                          4              ·                              M                sc                PUSCH                                              )                                    (        1        )            
In Equation (1), 0 is a number of HARQ-ACK information bits, βoffsetHARQ-ACK is a parameter informed to the UE through RRC signaling, Qm is a number of data information bits per modulation symbol (Qm=2, 4, 6 for QPSK, QAM16, QAM64, respectively), R is a data code rate of an initial PUSCH transmission for the same TB, MscPUSCH is a PUSCH transmission BW in a current subframe, and ┌ ┐ indicates the ceiling operation that rounds a number to its next integer.
The data code rate R is defined as shown in Equation (2).
                    R        =                              (                                          ∑                                  r                  =                  0                                                  C                  -                  1                                            ⁢                              K                r                                      )                    /                      (                                          Q                m                            ·                              M                sc                                  PUSCH                  -                  initial                                            ·                              N                symb                                  PUSCH                  -                  initial                                                      )                                              (        2        )            
In Equation (2), C is a total number of data code blocks and Kr is a number of bits for data code block number r.
The maximum number of encoded HARQ-ACK symbols is limited to the number of REs in 4 DFT-S-OFDM symbols (4·MscPUSCH). The nominal number of encoded HARQ-ACK symbols, Qnominal′, to achieve a target reception reliability is
      Q    nominal    ′    =            (              ⌈                              O            ·                          β              offset                              HAQR                -                ACK                                                                        Q              m                        ·            R                          ⌉            )        .  
The determination for the number of encoded HARQ-ACK symbols when the PUSCH conveys multiple TBs is similar to when the PUSCH conveys one TB.
An HARQ-ACK information bit is encoded as a binary ‘1’, if the TB is correctly received (ACK), or as a binary ‘0’, if the TB is not received or is incorrectly received (NACK). For HARQ-ACK information consisting of 2 bits [o0ACK o1ACK], with o0AcK and o1ACK the encoding is given in Table 1 where o2AcK=(o0ACK+o1ACK)mod 2 to provide a (3, 2) simplex code for the 2-bit HARQ-ACK.
TABLE 1Encoding for 1-bit and 2-bits of HARQ-ACKQmEncoded HARQ-ACK - 1 bitEncoded HARQ-ACK - 2 bits2[o0ACK y][o0ACK o1ACK o2ACK o0ACK o1ACK o2ACK]4[o0ACK y x x][o0ACK o1ACK x x o2ACK o0ACK x x o1ACK o2ACK x x]6[o0ACK y x x x x][o0ACK o1ACK x x x x o2ACK o0ACK x x x x o1ACK o2ACK x x x x]
For HARQ-ACK information consisting of multiple bits, as it may be the case for operation in a Time Division Duplex (TDD) system or when the UE is configured by the NodeB communication in multiple cells, the encoding can be, for example, using a block code such as a Reed-Mueller code. To account for the coding gain, a factor g(O) corresponding to the gain of block coding or simplex coding (transmission of O>1 HARQ-ACK bits) over repetition coding (transmission of O=1 HARQ-ACK bits) can be included in the determination of required coded symbols as shown in Equation (3).
                              Q          ′                =                  min          (                                    ⌈                                                O                  ·                                      β                    offset                                          HARQ                      -                      ACK                                                                                                            Q                    m                                    ·                  R                  ·                                      g                    ⁡                                          (                      O                      )                                                                                  ⌉                        ,                          4              ·                              M                sc                PUSCH                                              )                                    (        3        )            Alternatively, as an approximation, the coding gain factor g(O) may be absorbed in the parameter βoffsetHARQ-ACK. Then, Equation (3) is the same as Equation (1).
A UE may determine the number of HARQ-ACK information bits O depending on its operating environment. Examples are subsequently described.
For a Frequency Division Duplex (FDD) system and communication over a single DL cell, a UE may determine O based on the number of TBs it receives in a PDSCH.
For a TDD system and communication over a single DL cell, a UE may determine O based on the number of TBs for the configured TM and based on a number of PDSCHs that is either indicated by the UL SA scheduling the PUSCH transmission or, if such an UL SA does not exist, based on the maximum number of PDSCHs for which the UE may generate HARQ-ACK information in the PUSCH.
For an FDD system and UE configured communication over multiple DL cells, the UE may determine O based on the number of TBs for the configured TM in each configured cell and based on the number of configured DL cells.
For a TDD system and UE configured communication over multiple DL cells, a UE may determine O based on the number of TBs for the configured TM in each configured DL cell and based on a number of PDSCHs for each configured DL cell that is either indicated by the UL SA scheduling the PUSCH transmission or, if such an UL SA does not exist, based on the maximum number of PDSCHs for which the UE may generate HARQ-ACK information in the given PUSCH.
As HARQ-ACK requires the highest reception reliability among UCI types, the respective REs are located only in DFT-S-OFDM symbols next to the RS in each slot in order to obtain the most accurate channel estimate for HARQ-ACK demodulation.
FIG. 4 illustrates the multiplexing of HARQ-ACK REs in a PUSCH subframe.
Referring to FIG. 4, encoded HARQ-ACK bits 410 are placed in REs next to an RS 420 in each slot of a PUSCH subframe. The placement starts from the RE with the highest index (in a virtual frequency domain) and continues with lower indexed REs sequentially. The remaining portions of the subframe may convey data information bits 430.
When the number of the HARQ-ACK information bits is large and the Spectral Efficiency (SE) of the PUSCH transmission is low (as determined by the product of Qm·R), the number of encoded HARQ-ACK symbols Qnominal′ may exceed the upper bound of 4·MscPUSCH. In that case, the required HARQ-ACK reception reliability cannot be met.
The number of HARQ-ACK information bits can be reduced by HARQ-ACK bundling, which the NodeB can configure a UE to perform through higher layer signaling, e.g. when the UE has low UL SINR. HARQ-ACK bundling is a compression mechanism for the total number of HARQ-ACK information bits and can be in the spatial domain, the time domain, the cell domain, or a combination of these domains. Spatial domain bundling is applicable when a single PDSCH transmission conveys multiple TBs. Time domain bundling is applicable in TDD systems where a UE can receive multiple TBs through multiple PDSCHs in respective multiple DL TTIs. Cell domain bundling is applicable when a UE receives multiple PDSCHs from respective multiple cells in the same DL TTI.
For HARQ-ACK bundling in a given domain, a UE generates a single HARQ-ACK information bit corresponding to multiple TBs in that domain. If the UE correctly receives all of these multiple TBs, the HARQ-ACK information bit has the ACK value (binary 1). If the UE incorrectly receives (or does not receive) any of these multiple TBs, the HARQ-ACK information bit has the NACK value (binary 0).
Although conventional HARQ-ACK bundling can reduce the number of HARQ-ACK information bits even to a single bit, it is also highly suboptimal as it eliminates most of the HARQ-ACK information. For example, if a UE correctly receives all TBs except one, it generates a NACK, which will result in unnecessary retransmissions for all TBs except for the incorrectly received one. Moreover, as the NodeB cannot know which TBs the UE incorrectly received, the respective HARQ process cannot be applied with the correct Redundancy Version (RV). Additionally, HARQ-ACK bundling is configured to a UE by the NodeB through RRC signaling, it cannot account for dynamic changes in the number of available resources (REs) for HARQ-ACK multiplexing in the PUSCH.